All polycrystalline diamonds for tools have hitherto been produced by the sintering method. Namely, diamond powder--polycrystalline diamond granules--is charged in a mold, is put tinder high pressure, is heated to a high temperature and is kept For a certain time at the high temperature under high pressure. Diamond granules are coupled together by the action of the heat and the pressure. Thus powder is integrated into a body. The shape of the sintered body is determined by the mold. Sintered diamond has been used on cutting devices for nonferrous metals, drill bits and drawing dies.
For example, the Japanese Patent Publication No. 52-12126 disclosed a sintered body of diamond including cobalt by about 10 to 15 volume percent which was made by sintering diamond powder being kept in contact with the cemented carbide of group WC-Co (tungsten carbide-cobalt), diffusing some part of cobalt into the diamond powder as a binder metal. The sintered diamond body including cobalt as solid solution is gifted with a practical utility for cutting tools for nonferrous metals. In general, since diamond has the tendency to be alloyed with iron or steel, diamond tool is not used on cutting ferrous metals.
However, the sintered diamond has a drawback of the poor heat resistance. The abrasion resistance and the strength of the sintered body of diamond are greatly lowered by being heated above 700.degree. C. The sintered body of diamond is destroyed by being heated above 900.degree. C.
Single crystal diamond or polycrystalline diamond has high heat resistance. Why is the heat resistance of the sintered diamond so poor? One reason is that diamond is partly converted to graphite on the boundary between the diamond granules and the cobalt solid solution at high temperature. Conversion of diamond into graphite lowers the strength and the abrasive resistance, because graphite is weaker than diamond in strength. Another reason is that the difference of the thermal expansion between cobalt and diamond generates strong thermal tension at the boundary between the diamond granules and the cobalt solid solution.
To improve the poor heat resistance of the sintered body of diamond, the Japanese Patent Laying Open No. 53-114589 proposed a sintered body of diamond which is rid of cobalt as a binder metal by treating the sintered diamond body with acid. The sintered diamond body must be immune from the problems of the graphite conversion and the thermal tension which would occur at the boundary of the diamond granules and the cobalt solid solution, since the sintered body includes no cobalt. However, the sintered diamond body without cobalt becomes severely porous, because the spaces where cobalt has occupied are left as holes after cobalt is dissolved away by acid. Although the heat resistance is raised by washing cobalt away, the mechanical strength of the sintered body is lowered because of the porosity. The diamonds produced by the sintering method are accompanied by these drawbacks. The sintering method at present cannot satisfy the requirements of both mechanical strength and heat resistance.
Recently, a new technology which enables us to synthesize diamond polycrystals chemically from vapor phase has been developed. This technology is called a chemical vapor phase deposition (CVD) method or simply a vapor phase synthesis. The method comprises the steps of diluting hydrocarbon gas with hydrogen gas to less than 5 volume percent, introducing the mixture gas into a reactor under the pressure of several tens Tort (several thousands Pa), exciting the material gas for resolving it to an active state by some means and depositing diamond on the substrate heated at a certain temperature. With regard to the means for exciting the material gas, various means have been proposed, e.g. heating by filament heater or exciting the material gas by electrons or plasma. Some different CVD methods have been proposed according to the means for excitation of material gas.
The Japanese Patent Laying Open No. 58-91100 ('83) proposed a method comprising the steps of preheating the material gas by a hot electron emission material heated above 1000.degree. C., introducing the preheated material gas onto a heated substrate, resolving hydrocarbon to active states, e.g. ions, electrons and neutral radicals and depositing a diamond polycrystal on the substrate.
The Japanese Patent Laying Open No. 58-110494 ('83) proposed a method comprising the steps of exciting hydrogen gas into plasma by the microwave electrodeless discharge, mixing the plasma-excited hydrogen with hydrocarbon gas and depositing a diamond polycrystal on a heated substrate.
Thus, there are various kinds of the CVD methods for growing diamond crystals according to the means for excitation
Diamond polycrystals are grown by the CVD methods. There are two ways for applying the diamond polycrystals to tools. One is separating the diamond polycrystal from the substrate and fitting the diamond polycrystal to an end of a tool. The other is depositing the diamond polycrystal directly on an edge of a tool instead of a substrate. The edge of the tool is reinforced by the diamond coating.
Japanese Patent Laying Open No. 1-153228 ('89) and the Japanese Patent Laying Open No. 1-210201 ('89) proposed a method for producing a diamond tool comprising the steps of depositing a diamond polycrystal on a substrate by a chemical vapor deposition (CVD) method, etching the substrate away with acid or other pertinent solution and fitting the separated diamond polycrystal to an edge of a tool which is made of metal. However, the diamond tool consisting of a diamond edge and a metal body lacks chip resistance and abrasion resistance. The "chip resistance" is here defined as the strength for keeping its shape against an external shock without being cracked away. Low chip resistance means being likely to be cracked by an external shock. The "abrasion resistance" is here defined as the strength against abrasion. Low abrasion resistance means being likely to be abraded easily. Intrinsically diamond should have high abrasion resistance and high chip resistance, but the diamond crystal synthesized by the present CVD methods is not endowed with the high abrasion resistance and high chip resistance by some unknown reasons.
The tools whose edge is coated with polycrystalline diamond have also been proposed. On whole of a tool or on a part of a tool as a substrate, diamond polycrystal is grown by the CVD method. Since the edge of the tool is coated with diamond, the edge would have enough strength. However, the CVD coated tools in practice show poor performance--weak strength, low chip resistance and low abrasion resistance. The reason of the poor performance is partly because the diamond polycrystal is too thin, partly because the adhesion force between the diamond and the tool metal is insufficient and partly because the diamond is likely to peel off from the metal surface. However, it is difficult to heighten the adhesion strength, since the tool metal and the diamond are totally different with regard to many physical or chemical properties, e.g. crystal structure, conductivity, thermal expansion.
The Japanese Patent Laying Open No. 2-22471 ('90) proposed an improved CVD coated tool. The adhesion strength is heightened by coating a cemented carbide tool with an improved diamond compound. However, such a diamond coated tool often shows poor cutting performance dependent on the roughness of the object to be cut. Further, the cutting performance of the tool is totally insufficient, when it cuts the hard objects with high cutting resistance, e.g. Al--17% Si alloy (Al 83%, Si 17%), Al--25% Si alloy (Al 75%, Si 25%).
One purpose of the invention is to provide a polycrystalline diamond tool with high strength, high adhesion resistance, high heat resistance, and high abrasion resistance.
Another purpose of the invention is to provide a polycrystalline diamond tool which excels in chip resistance and abrasion resistance for cutting the hard objects with high cutting resistance.
Another purpose of the invention is to provide a method for producing a polycrystalline diamond with high strength, high adhesion resistance, high heat resistance and high abrasion resistance.